Except the big announcement at the wedding seems more likely to be that Nick is not the father of Gabi's baby and he has only just got confirmation of that.

Yes, a few days ago. He still had plent of time rather than quietly sitting through preparations and almost the entire ceremony before disrupting everything. Of course, that wouldn't be as soapy. But still, the guy could have confronted Gabi any time after he overheard her talking with Cameron.

My timelines could be out due to the lack of PBP's but I thought it was the Tuesday 8th of January show where Chad found out via overhearing Cameron that Nick wasn't Gabi's baby dadda.

When is the actual date for the wedding?

In days time it could be tomorrow.

It starts on Friday the 11th and the big reveal comes on Monday the 14th. Not sure what that will turn out to be in Days time, but according to the spoilers, there are at least a couple of times where Chad lurks in the background while wedding preparations are underway. Like I said, he's choosing to keep mum until the big moment.